Silicon semiconductor devices are employed in the detection of infra-red energy because the energy can penetrate into the silicon. In a typical detector, a back side of a silicon chip is directed toward a source of light energy which penetrates into the chip and is sensed on an opposite side of the chip which contains detection circuitry. The need to expose the backside of the chip presents special packaging problems which are not present in computer or communications chips where the back side is attached to the package.
Infra-red imaging devices are in some applications fabricated on silicon chips which are greater than one centimeter on a side. The chip and the package in which its is held must also operate at cryogenic temperatures. Differences in thermal expansion between the chip and the package as they descend from the temperature of bonding to minus 196 degrees Centigrade can cause stress in the chip which is a brittle material.
Various package designs for infra-red imaging devices are known. A first design employs a package shaped like a picture frame. A back side of a chip faces out from behind the picture frame and edges of the backside of the chip are bonded to an interior periphery of the picture frame. A front side of the chip is open to interior portions of the picture frame where wire bonds are made between the chip and the picture frame. Heat must be dissipated via a long path from the front side of the chip through the semiconductor material of the chip to the edges thereof which are bonded to the picture frame. Another problem with this design is that the area for back side illumination of the chip is reduced by the area needed for the bond to the picture frame. Another problem is alignment of the chip in the opening of the package.
A second design is used with "flip-chip" type of bonding where a chip having solder balls which are built upon the active circuit bearing side of the chip are bonding to a package. The solder balls are aligned indirectly to bonding pads on the package since the joint is hidden from view. Heat is applied to make the solder bond and a space typically remains between the chip and the package. Heat must be dissipated from the active elements along the chip and down through the solder bonds. One disadvantage of this structure is that the metal (solder) bonds which serve both to communicate signals and to mount the chip are subject to fatigue from thermal stress caused by differential thermal expansion between the package and the chip as both experience temperature changes. In addition, the costs associated with this type of design are higher than is desirable in some applications.
It is desirable to package an infra-red detecting semiconductor chip to accommodate the difference in thermal expansion between a chip and its package so as to reduce the mechanical stress applied to the chip during changes in temperature. It is also desirable to provide an efficient path for thermal conduction from active elements on the chip to the package which acts as a heat sink. It is useful to provide for ease of assembly by being able to observe locating indicia on the chip, and to assemble the chip to the package in a reproducible process with a minimum number of steps.